Developing approaches to induce and maintain robust cardiac allograft is the ideal solution to the morbidity and mortality associated with chronic immunosuppression. We uniquely demonstrated that kidney transplant tolerance in mice, swine and NHP remarkably facilitates heart transplant tolerance. Understanding the mechanisms underlying these observations has broad implications for the transplant community well beyond the relatively small numbers of patients likely to receive kidney/heart cotransplants. The deciphered mechanisms could guide novel therapeutic approaches to induce tolerance to heart grafts (among other tolerance-resistant organs) in the absence of a kidney, or improve outcomes without inducing tolerance. Elucidating mechanisms of kidney induced cardiac allograft tolerance (KICAT) is the focus of this Program. Our preliminary results in murine, swine, and NHP models implicate regulatory T cells (Treg) as the end effectors of KICAT with kidney-specific cells (e.g. plasmacytoid dendritic cells (pDC), renal tubular epithelial cells (RTEC)) and/or cell products (i.e. erythropoietin (EPO)) amplifying those regulatory mechanisms. Indeed, new data indicate that erythropoietin (EPO), a hormone produced by the adult kidney and formerly thought only to induce red blood cell development, mediates kidney tolerance by functioning as a Treg-enhancing immunosuppressant. Together our joint data support the hypothesis that: high local concentrations of EPO in the donor kidney graft directly inhibit pathogenic effector T cells and induce TGF? production by RTEC and kidney pDC that facilitate generation and stability of donor-reactive Tregs. These Treg crucially mediate heart graft tolerance. To test this hypothesis we have designed a Program consisting of 3 interactive Projects (2 at MGH, Boston and one at Mount Sinai, NY) that use murine and NHP models. P. Heeger (Mount Sinai, NY, Project 3) will test mechanisms of EPO-induced kidney transplant tolerance in mice. The Project will 1) determine the effects of kidney allograft-derived EPO on murine alloimmunity and allograft survival, 2) decipher the mechanisms through which EPO inhibits conventional alloreactive T cells, and 3) test the mechanisms of EPO on Treg induction and stability. R. Colvin and colleagues (MGH, Project 2) will use murine models of kidney and heart/kidney transplantation to 1) determine the general immunobiologic features of kidney induced systemic tolerance, 2) test whether KICAT is due to regulatory or deletional tolerance, and 3) test the hypothesis that specific kidney derived cells and mediators are responsible for tolerance induction. J. Madsen and colleagues (MGH, Project 1) will 1) characterize the overall robustness of the tolerant state induced by KICAT, 2) determine the role of regulatory T cells in KICAT, and 3) test the hypothesis that renal pDCs and EPO are required for KICAT. An immunopathology core (Core A) and the administrative core will support all 3 Projects. This Program is integrated such that early advances from mouse models will inform and refine the studies in NHP while clinical discoveries in NHP will be assessed mechanistically in the mouse.